Frequency conversion devices which use quasi-phasematching (QPM) in nonlinear optical waveguides are well known in the art. There are broadly two approaches used in order to establish the periodic structure required to achieve quasi-phasematching, those being chemical and electrical means. To date, the highest normalized conversion efficiencies for QPM second harmonic generation have been reported by Yamada et al, Applied Physics Letters, Vol. 65, p. 435 published Feb. 1, 1993, using electric field periodic poling in a waveguide in LiNbO.sub.3. The device and method of Yamada, however, exhibit a number of difficulties. Only very short interaction lengths have been reported, which indicates that an inhomogeneity is present in the devices. This is an important issue since the absolute conversion efficiency of this type of device increases as the square of the interaction length. A second serious problem with the device and method of Yamada is the requirement of very thin substrate dimension (&lt;200 .mu.m). The small thickness is necessary to avoid dielectric breakdown during the electric field poling step. Such thin substrates are extremely fragile and lead to low yields in fabrication and low mechanical robustness in the finished devices. A basic problem with the device and method, which relates to the inhomogeneity cited above, concerns the electric field configuration used to periodically pole the device. Yamada et al use a finger electrode structure atop the single crystal LiNbO.sub.3 wafer and a planar counter-electrode on the opposite face. A voltage applied between the top and bottom electrodes creates an electric field distribution having a spatially periodically varying magnitude beneath the finger electrode structure. The field is applied near room temperature. In some regions the field strength is high enough to reverse the ferroelectric polarization direction of the crystal, thus creating the periodically poled structure. In practice the process is difficult to control, there being a tendency for reversed domains to grow laterally during the poling process. This has required the use of pulsed field poling (pulsewidths in the vicinity of 100 .mu.s were reported by Yamada et al). The high coercive field for LiNbO.sub.3 (.about.20 V/.mu.m) demands multi-kilovolt pulses. The short pulse duration makes use of the field dependent kinetics of domain wall growth to try to control the poling process. The criticality of the process together with the relatively small modulation depth of the electric field strength as a function of position under the patterned electrode are problematic. When combined with inherent imperfections in material or fabrication, periodic structures which are sufficiently uniform to give only very short interaction lengths in the SHG process are the result. Yamada reports an interaction length of only 3 mm. The process would be much easier to control if the periodically varying electric field distribution had much greater modulation depth. Moreover, the criticality of the periodic poling procedure could be dramatically reduced if the electric field distribution was made to periodically change sign as well as magnitude beneath an electrode structure.
Matsumoto et al, Electronics Letters, Vol. 27, p 2040, published Oct. 24, 1991 discloses an electric field poling method and device for QPM frequency conversion. Matsumoto et al employs an interdigital electrode structure to create a periodic electric field, however the reference discloses a poling process which requires a temperature of 600.degree. C. or above. At temperatures employed by Matsumoto et al, several problems are attendant. First, at these temperatures, useful nonlinear materials conduct significant currents. Also, Matsumoto et al reports a corrugation on the nonlinear substrate surface caused by the poling process. Such a corrugation is undesirable since it results in waveguide power loss. It is well known that at temperatures employed by Matsumoto et al and particularly in the presence of an applied electric field, metal from electrodes can diffuse into the nonlinear substrate material. Such metal migration can alter the optical properties of the nonlinear material. In particular, a discoloration of the material beneath the electrodes has been reported. The change results in absorption losses in the nonlinear waveguide.